1. Field of the Invention
The present invention relates to electrostatic powder spray coating for producing evenly coated continuous films of thermoplastic and thermosetting powders on substrates or parts having electrically conductive geometrically complex surfaces and for enabling the use, without the occurrence of "cobwebbing," of significantly increased electrostatic charging voltages in electrostatic powder spray coating of preheated substrates and parts thereby to allow coating lines to be run faster.
2. Description of the Prior Art
With the increased attention given in recent years to cleaning up the environment, powder coating has increasingly come into use as a better solution than painting to protective, decorative and insulating problems. Powder coating has eliminated to a significant extent solid waste disposal and solvent emissions problems that are encountered with painting. This has resulted in much easier compliance with both state and federally imposed environmental regulations as well as a reduction in energy costs because of a reduced need for venting. Additionally, almost all of powder overspray that occurs can be recovered and reused. The remaining one or two percent that cannot be recovered can easily be disposed of as a solid. Another significant advantage of powder coating is that it provides one coat coverage without runs and sags.
A number of methods have been employed in the prior art for applying powder coating materials. These include a fluidized powder bed process, an electrostatic fluidized bed process, and electrostatic powder spraying. Other known electrostatic powder application methods include an electrostatic powder disc drive and an electrostatic chamber.
In the fluidized bed process, a preheated part or substrate is dipped or immersed in a fluidized bed, the powder being fluidized by introducing compressed air through a porous membrane into the powder bed. Mechanical vibration of the powder bed may also be used to enhance fluidization. Upon coming into contact with a preheated part, the powder particles melt and adhere thereto. Post heating is required if the residual heat of the part is insufficient to cause the adhered powder particles to flow and cure.
The electrostatic fluidized bed process utilizes a fluidized powder bed and electrostatic charging electrodes to create a cloud of electrostatically charged powder particles in a chamber above the powder bed. The electrostatically charged powder particles, which are all charged alike, repel each other and move upward in the chamber. Grounded substrates or parts to be coated are dipped or immersed in the resulting cloud of charged powder particles. Preheating of the part is not necessary with this method of powder coating since the forces of electrostatic attraction cause the deposition of powder on the surface of the part irrespective of whether it is hot or cold. If the part is preheated, the powder particles will melt upon contact therewith and adhere. If the part is cold, the charges on the powder particles hold them in place until the part is post heated.
Electrostatic powder spray coating is a widely used method of applying powder coating materials to grounded substrates or parts. In this method of powder coating, powder coating particles are supplied to an actuatable spray gun and are electrostatically charged in the nozzle region thereof. Upon actuation of the spray gun, a cloud of comminuted charged powder particles is discharged from the gun. The gun is used to direct the cloud of charged powder particles toward the grounded substrate or part to be coated. The propelling force is provided by the electrostatic forces of attraction between the charged particles and the grounded part to be coated and by the air that is used to transport the powder from a supply thereof to the spray gun and to effect the discharge of such powder therefrom. Powder particles deposited on the part are held thereon by electrostatic forces. The coated part is then subjected to heat, in an oven, for example, where the layer of unfused powder particles melts, flows and cures to provide a continuous film.
A more detailed discussion of the foregoing methods of powder coating including those involving an electrostatic powder disc and an electrostatic chamber is given in the "Users Guide to POWDER COATING" published by the Association for Finishing Processes of the Society of Manufacturing Engineers, one SME Drive, Dearborn, Mich. 48121. Also, an in-depth discussion of the basic processes involved in electrostatic powder spray coating is given in the paper entitled "Surface films produced by electrostatic powder deposition" by A. W. Bright, Department of Electrical Engineering, University of Southampton S509 5NH, published in the J. Oil Col. Chem. Assoc. 60, 23-27.
The electrostatic powder spray guns most widely used commercially are basically of two types, specifically external charging and internal charging. In external charging powder spray guns, a charging electrode is located at the front or tip of the nozzle of the spray gun. This electrode, when energized with a high voltage, low amperage direct electrical current, produces an ion cloud or corona in the region of the electrode. Each of the powder particles is exposed to this ion cloud as it passes through it and thus has a charge placed thereon. The charge placed on the powder particles is usually negative with respect to ground potential although, if desired, it may also be positive. The charged powder particles form a charged powder particle cloud in front of the spray gun, which cloud is attracted to and moves toward the grounded substrate or part to be coated. This movement is assisted by air currents resulting from the operation of the spray gun and results in the powder being deposited on the part.
In internal electrostatic charging powder particles, the powder particles are passed through an ionizing field or corona that is produced between electrodes mounted internally of the spray gun. The charged powder particles discharged from the nozzle of the spray gun form a cloud in front of the gun, which cloud is attracted to the grounded substrate or part, assisted by air currents resulting from the spray gun operation.
Irrespective of the type of electrostatic powder spray gun employed, subsequent heating of the powder particle coated substrate or part melts and cures the powder particles to provide a continuous film.
Attempts in the prior art to speed up the electrostatic powder coating spraying operation by preheating the substrate or part to be coated and thereby eliminating the need for a separate subsequent heating and curing step have been beset with problems. One of these problems is the occurrence of a phenomena termed "cobwebbing", as illustrated in FIG. 1 of the drawings. As shown in FIG. 1, heated powder particles appear to coalesce and form cobweb-like strings that float and drift about the substrate or part to be coated and deposit thereon or subsequently drop to the floor of the spray booth. Such coalesced strings or cobwebs are not recoverable. The occurrence thereof seriously impairs the appearance of the surface of the coated part and the efficiency of the electrostatic powder spray system. Additionally, the presence of such coalesced powder strings in the oversprayed powder comprises a serious impediment to the operation of the recovery system that is employed to reclaim such oversprayed powder. Specifically, such coalesced strings tend to clog the filter employed in the recovery system as well as the supply lines to the nozzle, and the nozzle itself, of the electrostatic powder spray gun.
A solution proposed in the prior art for the cobwebbing problem is to reduce the voltage on the charging electrode of the electrostatic powder coating spray gun from a voltage level in the range of 60-80 kilovolts (KV.), or perhaps higher, to a level of about 20 KV. While successful in reducing or suppressing cobwebbing in the electrostatic powder spraying of preheated substrates or parts, such reduction in voltage is undesirable because it significantly reduces the coating speed and requires slowing down of the powder coating lines.
Another solution employed in the prior art to solve the cobwebbing problem involves the addition of a siliceous ingredient specifically fumed silica, to the coating powder. While successful in reducing or suppressing cobwebbing in electrostatic powder spraying of preheated substrates or parts, the use of fumed silica, typically in the amount of 0.7 to 1.0%, is undesirable since it makes it harder to charge the powder particles and more difficult to deposit powder particles on the part to be coated. Additionally, the siliceous additive has been found hard to control in the manufacture of the powder and particularly in the overspray that is recovered and reused. In the absence of reasonably precise control, the appearance of the coating produced on the surface of the part being coated may be adversely affected with some areas appearing to be rough and dry.
Another problem that has been encountered in the prior art with electrostatic powder spraying of substrates or parts, particularly those with complex geometrical surfaces, is difficulty in effecting substantially even or uniform powder deposition in the inner portions of recesses and corners and associated side walls. This difficulty is widely believed to be due to a so-called "Faraday cage" effect. This effect is believed to involve repulsion of incoming charged powder particles by each other and by the charged layers of powder particles on opposing side walls. Such charged layers are believed to form before any charged powder particles can enter the inner portions of the recessed regions. It has been necessary, as a result, to expose the substrate or part for a longer time to the cloud of charged powder particles in the electrostatic powder spray to ensure that inner recessed areas and corner portions receive a powder coating layer. As a result, the thickness of the coating formed on outer portions of the associated side walls of recesses and corners has been found to be four or five times that of the coating on inner portions thereof.
By the terms "substantially uniform coating thickness" or "substantially even coverage," as used herein is meant a coating thickness on those portions of the substrate or part which are not subject to Faraday cage effects that is two times (or less) than that of the portions of the part that are subject to Faraday cage effects, and the thickness of the two portions can even be equal.
It is noted, as mentioned on page 79 of the aforementioned "User's Guide to POWDER COATING", that theories relating to internal charging of electrostatic powder spray guns claim the elimination of free ions in the charging process improves the deposition of powder in areas in which the "Faraday cage" effect inhibits coating. Irrespective of the validity of this claim, it is significant that no suggestion is made that internal charging provides a solution to the problem of an even powder coating on substrates and parts having geometrically complex surfaces.
Thus, there exists a need and a demand in the art for improvements in methods of and apparatus for electrostatic powder spraying for coating heated substrates or parts having complex geometrical surfaces to the end of avoiding cobwebbing while allowing the use of high electrostatic charging voltages and thereby allowing the coating lines to run faster, and achieving, also, uniform coating in inner portions of recessed regions or corners and the associated side walls. The present invention was devised to fill the technological gap that has existed in these respects.